Second EAGE Eastern Mediterranean Workshop

Triassic evaporites on- offshore western Greece are of great significance for the tectonic evolution of the sedimentary basins and the accumulation of hydrocarbons. They build the basis of the Alpine nappes and are the lubricants for the westwards motion of these units. They separate the upper (Alpine) from the lower (Preapulia) part of the sedimentary sequence and are the seals and traps of hydrocarbons generated during the Triassic. These basins have significant thickness (8 to 12 km) and are difficult to penetrate and map with conventional seismic methods. We have explored them with wide aperture techniques. The backstop areas to the west are composed of stretch continental crust. Sedimentary basins are much thinner than those of western Greece and the Triassic evaporites are absent. This is also the case for the basins southwest of Crete. Sediments range from 3 to 6 km and Triassic evaporites are absent. Structures of interest for oil exploration southwest of Crete occur at a distance of approx. 150 km from the coast, at the Mediterranean Ridge. Sediments are very thick (10 to 14 km) and are strongly tectonized. The existence of mud volcanoes is a direct indicator for hydrocarbon systems.

A regional structural section of 120 km with NE-SW orientation has been constructed in the Dinaride-Hellenide fold and thrust belt. Due to the limited availability and low quality of the seismic sections, an integration of gravity and magnetotelluric datasets have allowed to reduce the uncertainty of the interpretation. The gravity model allowed to test different structural scenarios until a satisfactory fit between the observed field values and the calculated values from the model was achieved. This model has been sensitivity tested based on different densities obtained from well data and surface geology. The magnetotelluric method, which was especially valuable in this area due to the resistivity contrast between the different formations of the area, allowed to define the geometry of the area around an elongated evaporitic body. Both methods indicate the limited depth of this evaporates and the presence of carbonates or clastics underneath.

We describe the main results of a Velocity Model Building (VMB) and a Prestack Depth Migration (PSDM) project targeting the imaging of a carbonate build-up in a pre-salt complex setting in the deep water of the Egyptian offshore. The purpose of the project was to support the characterization of the exploration targets and the subsequent optimization of the appraisal and development phases in the shortest possible turnaround times.

The heavy usage of HPC (High Performance Computing) resources enabled the fast generation of several scenarios used to iteratively refine the Top and Base of Salt interpretations in a close-loop integration with halokinetics specialists.

The applied workflow led to significant improvements of the highly complex velocity model that increased the quality of pre-salt imaging and the geological reliability of the reservoir structure, with a consequent de-risking of the appraisal and development wells.

During the last decade, the exploration targets offshore Egypt have moved subsalt. Messinian salt is a complex interval that changes its character going from West to East Mediterranean. Existing seismic data was not able to properly capture the pre-Messinian targets. We have used the available seismic and the non-seismic data to build the representative 3D Earth model, incorporating typical challenges offshore Egypt. 3D Finite Difference modeling was used to simulate different acquisition approaches. Based on the modeling results, further in this abstract we will show the best acquisition approaches to illuminate subsalt. Moreover we will examine the impact of the broadband seismic data as well as the impact of surface and internal multiples, inaccurate velocity models and absorption on the image below the salt.

Several areas of Earth with large accumulations of oil and gas also have huge deposits of salt below the surface and identifying where large salt deposits are precisely is very difficult. Currently seismic imaging still requires expert human interpretation of salt bodies. This leads to very subjective, highly variable renderings therefore to potentially dangerous situations for oil and gas company drillers. Deep learning algorithms have been used to solve several subsurface imaging tasks such as classification and segmentation.

These algorithms are part of the concept of automatic seismic interpretation (ASI), which is now enabling seismic interpreters to complete routine interpretation tasks much more efficiently than what could be done using legacy software. So, how do these ASI networks work when the salt identification task is considered? How efficient are they? What is the computational cost? How good are their outputs? How can we measure their performance and the value they add?

The deepwater biogenic gas discoveries in the Eastern Mediterranean have unique characteristics, especially due to the Messinian Salinity Crisis (MSC) between 5.96 and 5.33 Ma during which most of the Mediterranean and connected basins lost all or most of their water coverage. This resulted in geologically rapid changes in depositional environments and in the surface and subsurface conditions.

The role of the MSC in the formation and preservation of hydrocarbon accumulations in the area is an ongoing discussion. What are the relationships, if any, to the MSC? Are there features that are unique and limited to areas affected by the MSC? How does the event control the risks associated with the generation, migration, accumulation, preservation and loss of the hydrocarbons? What do we know about the charging conditions and mechanisms? Are there controlling processes that occur on very short geological time scales?

We use analogues and petroleum systems modeling to investigate hypotheses related to the generation, migration and entrapment of biogenic gas fields in connection with the MSC. A conceptual 2D petroleum systems model of the Zohr discovery is constructed based on publicly available data, and used to constrain our understanding of the properties and processes in the petroleum systems.

Sub-salt plays present an attractive exploration concept in the External Hellenides (Western Greece) but have never been substantiated. We present recent advances in “seeing through the salt” using examples from the Patraikos offshore block. We employed gravimetric-magnetic data analysis to elucidate subsurface structures of those areas where Triassic evaporites create an obstacle for seismic imaging and where depth-to-targets remains poorly defined. We evaluated three structural hypotheses via 2.5D gravimetric-magnetic forward modelling: 1) tectonically thickened evaporites, 2) involvement of a pre-salt-stratigraphy and underlying basement, 3) sub-salt imbricates of carbonate units. The outcome is not favorable towards overthickening of the Triassic evaporites in the subsurface of the Patraikos block. Instead, a pre-salt stratigraphy or sub-salt carbonate units at drillable depths better satisfy the gravimetric-magnetic data and allow for attaining a close match between the measured and theoretical gravimetric/magnetic responses. In terms of prospectivity, this result supports a sub-salt play: sub-thrust carbonates (more optimistic) or pre-salt stratigraphy and basement (less optimistic). Given the possible existence of similar situations along the whole external front of the Hellenides, this may result into a new play regionally widespread, thus contributing to a strong upgrade of the exploration potential of Western Greece.

Hydrocarbon exploration in structurally complex areas such as Western Greece requires good understanding of the main uncertainties controlling the petroleum systems and its associated risks. In the case of the external zones of the Hellenides Fold Belt such complexities are dominated by the presence of Upper Triassic evaporitic units that represent the regional detachment of the thrust sequences. The presence of salt in this fold belt exerts an important control on the structural style and, therefore, the structural play type. In basin and petroleum systems analysis, the reconstruction of the burial history is decisive to assess the quality of source, reservoir and seal rocks. With this purpose, and in order to better understand the main uncertainties present in the area of study, sensitivity analysis of the eroded thicknesses occurred in the Ionian zone during compression and uplift in the Miocene has been performed using maturity data at different locations. These erosion estimates allow constraining the restoration of structural sections and a better 2D analysis of the burial history of the area. In the Ionian zone, and due to tectono-stratigraphic variations from north to south, the assessment allows discerning between areas with different timing of hydrocarbon generation and potential trap charge.

Apulian Platform has been developed on the passive margin of Africa, the Adria or Apulia margin. It is characterised by platforms and deep-sea basins controlled by the Norian-Liassic rifting phase of the Tethyan rifting (Bertottti, 1993). The Eratosthenes carbonate platform sits on top of a thicker continental block (Moho depths of 32–35 km, Feld et al. 2017) compared to adjacent basins and would thus have been subject to lower rates of subsidence than the Levant Basin ( Gardosh et al., 2010 ). This differential subsidence between crustal segments of different nature is well described for other Mediterranean passive margins.

The Eratosthenes and Apulian platforms remain wholly carbonate (“clean”). Seismic interpretation suggests that the infilling adjacent to both the Eratosthenes and the Apulia carbonate platform consists of gravity and mass transport complexes (MTCs) as well as deep pelagic sediments that onlap the paleo-slopes of the Mesozoic platforms. The mechanisms that triggered these large slope failures (maximum width 15–30 km) could be a combination of several parameters including (a) rapid sedimentary loading that can generate excess pore pressure, (b) earthquakes associated with the major geodynamic events, and (c) erosion of older successions due to fluctuations in sea level.

The Levant Basin is currently a hot-spot in terms of oil and gas exportation. The world-class gas discoveries offshore Egypt, Cyprus and Israel within Oligo-Miocene turbiditic sands and Early Cretaceous carbonate buildups turned the limelight to this part of the East Mediterranean. Open questions regarding its tectonic evolution include the depth, thickness and nature of the crust, the dominant trend of the Tethyan rift structures and phases, the effect of the Late Mesozoic plate collision along the Cyprus and Latakia Arcs and the distribution, nature, and origin of the Early–to-Middle Miocene structures. Data collected in both the Levant northern and southern sub-basins allow comparing them and drawing conclusions regarding few of these questions. Both basins lack any sign of the Late Mesozoic collision, they subsided fast since the Early Tertiary and were subject to intense Early-to-Middle Miocene activity, resulting in diapir-like structures in the south and long, N-S folds in the north. The Oligo-Miocene section in both basins is as yet a major target for oil and gas exploration and the implications of our study for the timing of trap formation coupled with hydrocarbon expulsion and migration open new directions for future studies.